1. Field of the Invention
The present invention relates in general to a combined method of, making a serially connected organic electroluminescent device and, more particularly, to a method for improving color-shift of the serially connected organic electroluminescent device.
2. Description of the Related Art
Use of organic electroluminescence device (OELD) in the flat panel displays possesses several competitive advantages, such as self illumination, high brightness, wide viewing angle, vivid contrast, quick response, broad range of operating temperature, high luminous efficiency and uncomplicated process of fabrication. Thus, the OELD represents a promising technology for display applications and has receives the worldwide attention in the recent years.
FIG. 1 (related art) illustrates a conventional structure of organic electroluminescence device. The typical structure of OELD 1 is mainly constructed by interposing an organic light emitting layer 15 between an anode (formed on a substrate 10) 11 and a cathode 19. The material of the organic light emitting layer 15 is able to emit light when excited by electric current. A hole transport layer (HTL) 13 is interposed between the anode 11 and the organic light emitting layer 15. Also, a hole injection layer (HIL) (not shown) could be optionally sandwiched between the HTL 13 and the anode 11. An electron transport layer (ETL) 17 is interposed between the cathode 19 and the organic light emitting layer 15. This laminated structure of OELD facilitates the electron flow from the cathode 19 to the anode 11. The organic light emitting layer can be divided into tow groups according to the materials in use. One group is a molecule-based light emitting diode, substantially comprising the dyestuffs or pigments, and so called as “OLED” (i.e. organic light emitting diode) or “OEL” (i.e. organic electroluminescence). The other group is a polymer-based light emitting diode, so called as “PLED” (i.e. polymer light emitting diode) or “LEP” (i.e. light emitting polymer).
The luminance efficiency of a single color OELD has been successfully improved. For the worldwide OELD manufacturers, however, it is the final goal to provide an OELD with full-color emission. The OELD with full-color emission includes replicated pixels, and each pixel is able to emit red, green and blue light.
The smaller the pixel size, the higher resolution the OELD. “Full-color emission” display can be prepared by various methods. The first method is achieved by placing red, green and blue OELDs in a side-by-side configuration within a single pixel, for separately emitting three essential colors (Red, Green, Blue), and any combination thereof. The second method is achieved by a combination of a white light-OELD and a color filter (CF). The third method for providing full-color emission is the use a blue emitter and a color changing media (CCM) (such as a color conversion film). The blue light is converted to green or red by the color-changing media (CCM). The first and second methods are the most direct methods used in the manufacture.
Although optimal light efficiency of an OELD can be achieved by the RGB side-by-side method, it makes the fabrication more complicated that the shadow-masks required for depositing different emitters to create pixels with red, green, and blue subpixels. Also, the color purity and the luminance efficiency are important parameters and need to be well controlled. Besides, the OELD fabricated by the RGB side-by-side method possesses the disadvantages, such as different voltages for driving the RGB sub-pixels being required, color of OELD not contrasted clearly and the difficulty of improving the resolution of OELD. Therefore, researches about the second method for fabricating full-color emission OELD (i.e. white light and CF) become thriving nowadays.
In the second method, a white electroluminescent material is deposited on the substrate, and a color filter is overlaid onto the white emitter array. Light emitted from the white electroluminescent material passes through the color filter to create the red, green, and blue subpixels. The red, green, and blue subpixels are controlled by the different thin film transistors (TFT) to present the colors shown on the display. The second method without the use of shadow-masks simplifies the process, and is the easier way to make a high resolution and clear contrast color OELD. Since the combination of a white electroluminescent material and a color filter does not require precise alignment as rigorous as pixelized OELDs, the production yield can be increased. Many combinations of organic electroluminescent units such as two-element and three-element structures can be used for making a white OELD. The light emitted from the organic electroluminescent units passing through the color filter yields light that appears purer blue, red and green colors. Although the approach of the white electroluminescent material and the color filter does possess many advantages, it is difficult to control the color purity and the resulting white color is generally behind the expectation.
FIG. 2A (related art) shows the electroluminescence (EL) spectra of a conventional OELD with two-element structure. It is assumed that the organic light emitting layer of the conventional OELD having EL spectra of FIG. 2A comprises a blue electroluminescent material and a yellow electroluminescent material, and a hole barrier layer is sandwiched there between. An electrical potential is applied between the anode and the cathode to make the OELD produce white light. According to the results of FIG. 2A, the EL spectra of this OELD depend on the electrical potentials. In FIG. 2A, the OELD having higher luminance (in units of cd/m2) is operated by higher electrical potential. Slightly blue white light (i.e. stronger intensity shown in the shorter wavelength) is emitted from the OELD when a low or normal electrical potential is applied between the anode and the cathode. However, slightly yellow white light (i.e. stronger intensity shown in the longer wavelength) is emitted from the OELD when a high electrical potential is applied.
FIG. 2B (related art) shows the electroluminescence (EL) spectra of a conventional OLED with three-element structure. It is assumed that the organic light emitting layer of the conventional OELD having EL spectra of FIG. 2B comprises a yellow electroluminescent material sandwiched between two blue electroluminescent materials. An electrical potential is applied between the anode and the cathode to make the OELD produce white light. According to the results of FIG. 2B, the EL spectra of this OELD depend on the electrical potentials. Similarly, the OELD having higher luminance (in units of cd/m2) of FIG. 2B is operated by higher electrical potential. Contrary to the result of FIG. 2A, slightly yellow white light (i.e. stronger intensity shown in the longer wavelength) is emitted from the OELD of FIG. 2B when a low or normal electrical potential is applied between the anode and the cathode. Slightly blue white light (i.e. stronger intensity shown in the shorter wavelength) is emitted from the OELD of FIG. 2B when a high electrical potential is applied.
Both of the color-shift conditions of FIG. 2A and FIG. 2B causes impure white light, and the light color is behind the expectation. Therefore, “color balance” is one of the important issues in the full-color emission technology achieved by the combination of the white electroluminescent material and the color filter.